1. Field of the Invention
The present invention relates to a method of producing mesophase pitch based carbon fibers and a nozzle for spinning the base pitch fibers. More specifically, it relates to a method of producing carbon fibers having no cracks along the fiber axis and having an improved mechanical strength, and a nozzle suitable for use with this method.
2. Description of the Related Arts
Carbon fibers are generally classified into two groups; PAN (polyacrylonitrile) based carbon fibers and pitch based carbon fibers. The pitch type carbon fibers drived from a mesophase pitch with a high mesophase content can have a higher modulus of elasticity.
The conventional mesophase pitch based carbon fiber usually has cracks along the fiber axis when the fiber has a higher modulus of elasticity and a high degree of graphitization. Such the carbon fiber, of course, is of little value in practical use.
The crystallite orientation of the mesophase pitch based carbon fiber in the cross-section transverse to the fiber axis (hereinafter referred to as "fiber cross-sectional structure)" varies in accordance with the spinning conditions. There are basically three types of fiber cross-sectional structure; i.e., an onion type in which a graphite crystallite is crystallized along coaxial circles in the fiber cross-section, a radial type in which the crystallites are radially oriented about the center of the fiber cross-section, and a random type in which the crystallites are distributed in a random manner without any regularities. Existing fibers, however, often have an intermediate structure in which these types are mixed. Also, there may be defects in the carbon fiber, such as, longitudinal splits, cracks or voids throughout the fiber length. Note, these various defects and fiber cross-sectional structures are closely related to the quality of the carbon fiber.
Nozzles have been proposed for obtaining pitch fibers having the onion type and random type fiber cross-sectional structures free from cracks along the fiber axis. For example, a nozzle described in Japanese Unexamined Patent Publication (Kokai) No. 59-168127 has a capillary with a cross-section widened or constricted widened at an exit thereof. Another nozzle disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-163424 has a capillary with a non-circular cross-section, by which a modified radial type or random type pitch fiber is obtained. Alternatively, U.S. Pat. No. 4,376,747 discloses a nozzle with a filter material at the upper portion of a capillary. Japanese Unexamined Patent Publication (Kokai) No. 60-259609 also discloses a nozzle with a molded element set at the upper portion of a capillary. Japanese Unexamined Patent Publication (Kokai) No. 61-258024 proposes a provision of a mesh-like layer in a region upstream of a capillary. A carbon fiber in which the degree of graphitization is restricted in the direction transverse to the fiber axis, and a process for the production thereof, are disclosed in Japanese Unexamined Patent Publication (Kokai) Nos. 62-104927 and 62-177222. According to this process, a pitch used for spinning a pitch fiber is mechanically stirred just above a capillary, by which mesophase regions are is finely divided into smaller sizes and randomly oriented. Japanese Unexamined Patent Publication (Kokai) No. 63-211325 discloses a nozzle having more than two capillaries for one counter-bore, so that a pitch flow is made turbulent and the resultant carbon fibers can be free from cracks along the fiber axis.
As stated above, there are many proposals for preventing cracks in the mesophase pitch based carbon fiber and for improving the qualities thereof. These proposals, however, have drawbacks, such as a complex nozzle structure, difficulty of uniform insertion of various tools in a plurality of counter-bores, and a lack of a uniform heat transmission, due to the formation of a large space within a nozzle. Therefore, the conventional nozzles are disadvantageous in that at least one of the spinning stability, the rigidity, the manufacturing cost, and the ease of nozzle cleaning are unsatisfactory, and cannot satisfy users' requirements.